Deepwater hydrocarbon production requires that significant obstacles be overcome, especially in the area of transfer of the various produced fluids. There are several types of flowlines or “risers” which can be used to enable this fluid transfer. For drilling and production purposes, the offshore body of water can be thought of as having two zones whose characteristics control which type of risers are practical therein. The wave zone, within approximately 100 meters of the surface, is characterized by the continuous motion and substantial forces which vessels and risers passing through the zone experience, due to the effects of near surface conditions such as wind, tides, and currents. These constant motions and forces exert fatigue-inducing stresses upon risers that traverse the wave zone, especially rigid risers. Therefore, flexible risers are best suited for use within the wave zone. In the deepwater zone, approximately 300 meters from the surface and deeper, the constant motions characteristic of the wave zone are substantially reduced; instead this zone is characterized by significant hydrostatic pressure which risers therein must withstand.
There have been several different riser systems proposed for use in deepwater hydrocarbon production. Some of these systems attempt to use a single type of riser, and others combine different riser types to enable fluid communication throughout both the wave and deepwater zones. Each of these methods has shortcomings which are overcome by the present invention.
Two methods have been proposed which were designed to overcome the difficulties of deepwater production while using a single type of riser. For example, one system involves the use of a flexible riser system from the production pipelines or subsea manifold on the marine bottom to the floating facilities. The major limitation of this method is that in order to withstand the hydrostatic pressure and high tensile loads present in the deepwater zone, these flexible risers are limited to relatively small interior diameters.
Another deepwater production method, that also teaches the use of a riser system with a single riser type, involves the use of steel catenary risers (SCRs). In this method a steel pipeline is laid along the sea floor and curved gently upward in a catenary path through the wave zone and connected directly to the floating vessel on the surface. The disadvantages inherent in this method are that: 1) the weight of such a steel catenary riser system must be borne by the floating vessel; 2) the steel catenary risers must be thickened to withstand the effects of the wave zone {which results in even more weight}; 3) the steel catenary risers are still subject to fatigue caused by the near surface effects, which could necessitate large-scale repairs which would be very difficult and expensive because of the depths at which they must be performed.
Deepwater hydrocarbon production therefore lends itself readily to a riser system employing two different types of risers, one set of risers designed to withstand the hydrostatic pressures of the deepwater zone and the other set of risers designed to withstand the constant and varying forces and motions of the wave zone. Two methods have been proposed which were designed to overcome the difficulties of deepwater production with riser systems that employ two different types of risers. The first such method, referred to as a hybrid riser tower, consists of a rigid section which extends vertically from the sea floor to a fixed position below the wave zone and a flexible section which is comprised of flexible pipe flowlines (“jumpers”) that extend from the top of the rigid section, through the wave zone, to a floating vessel on the surface. A submerged buoy is typically used to maintain the rigid section of the hybrid riser tower in a substantially vertical position.
The other two-type riser system consists of steel catenary risers and flexible pipe jumpers used to enable fluid communication between the sea floor and the surface of a body of water. In this method, a submerged buoy is used to support the upper end of the SCR(s) at a location substantially below the wave zone. Flexible pipe jumpers extend from the top of the rigid (SCR) section, through the wave zone, to a floating vessel on the surface.
By using risers designed to withstand the characteristics of the two zones encountered in deepwater hydrocarbon production, both of these two-type riser systems are improvements over the single type riser systems discussed above. There remains, however, a need for a riser system that allows both local and remote fluid communication in deepwater applications.